Katja Heinicke

Abnormal haemodynamic response to exercise in heart failure with preserved ejection fraction

Peak oxygen uptake (VO2) is diminished in patients with heart failure with preserved ejection fraction (HFpEF) suggesting impaired cardiac reserve. To test this hypothesis, we assessed the haemodynamic response to exercise in HFpEF patients.

Effect of changes in fat availability on exercise capacity in McArdle disease

BACKGROUND The major fuel for exercising muscle at low exercise intensities is fat. OBJECTIVE To investigate the role of fat metabolism in McArdle disease (also known as glycogen storage disease type V), an inborn error of muscle glycogenolysis, by manipulating free fatty acid availability for oxidation during exercise. DESIGN Randomized, placebo-controlled, crossover trial. SETTING Hospitalized care. PATIENTS Ten patients (8 men and 2 women) with McArdle disease. INTERVENTIONS Patients cycled at a constant workload corresponding to 70% of their maximum oxygen consumption. In random order and on separate days, patients received nicotinic acid (a known blocker of lipolysis) to decrease the availability of free fatty acids or 20% Intralipid infusion to increase free fatty acid availability during exercise. Results were compared with placebo (isotonic sodium chloride solution infusion) and glucose infusion trials. MAIN OUTCOME MEASURES Exercise tolerance was assessed by heart rate response to exercise during different infusions. RESULTS Free fatty acid levels more than tripled by Intralipid infusion and were halved by nicotinic acid administration. Heart rate was significantly higher during exercise in the Intralipid infusion and nicotinic acid trials compared with the placebo and glucose infusion trials, an effect that was observed before and after the patients had experienced the second wind phenomenon. CONCLUSIONS Lipids are an important source of fuel for exercising muscle in McArdle disease, but maximal rates of fat oxidation seem limited and cannot be increased above physiologically normal rates during exercise. This limitation is probably caused by a metabolic bottleneck in the tricarboxylic acid cycle due to impaired glycolytic flux in McArdle disease. Therapies aimed at enhancing fat use in McArdle disease should be combined with interventions targeting expansion of the tricarboxylic acid cycle.

The effect of rowing ergometry and resistive exercise on skeletal muscle structure and function during bed rest

Exposure to microgravity causes functional and structural impairment of skeletal muscle. Current exercise regimens are time-consuming and insufficiently effective; an integrated countermeasure is needed that addresses musculoskeletal along with cardiovascular health. High-intensity, short-duration rowing ergometry and supplemental resistive strength exercise may achieve these goals. Twenty-seven healthy volunteers completed 5 wk of head-down-tilt bed rest (HDBR): 18 were randomized to exercise, 9 remained sedentary. Exercise consisted of rowing ergometry 6 days/wk, including interval training, and supplemental strength training 2 days/wk. Measurements before and after HDBR and following reambulation included assessment of strength, skeletal muscle volume (MRI), and muscle metabolism (magnetic resonance spectroscopy); quadriceps muscle biopsies were obtained to assess muscle fiber types, capillarization, and oxidative capacity. Sedentary bed rest (BR) led to decreased muscle volume (quadriceps: -9 ± 4%, P < 0.001; plantar flexors: -19 ± 6%, P < 0.001). Exercise (ExBR) reduced atrophy in the quadriceps (-5 ± 4%, interaction P = 0.018) and calf muscle, although to a lesser degree (-14 ± 6%, interaction P = 0.076). Knee extensor and plantar flexor strength was impaired by BR (-14 ± 15%, P = 0.014 and -22 ± 7%, P = 0.001) but preserved by ExBR (-4 ± 13%, P = 0.238 and +13 ± 28%, P = 0.011). Metabolic capacity, as assessed by maximal O2 consumption, (31)P-MRS, and oxidative chain enzyme activity, was impaired in BR but stable or improved in ExBR. Reambulation reversed the negative impact of BR. High-intensity, short-duration rowing and supplemental strength training effectively preserved skeletal muscle function and structure while partially preventing atrophy in key antigravity muscles. Due to its integrated cardiovascular benefits, rowing ergometry could be a primary component of exercise prescriptions for astronauts or patients suffering from severe deconditioning.

Recurrent myoglobinuria in a sporadic patient with a novel mitochondrial DNA tRNAIle mutation

The differential diagnosis of myoglobinuria includes multiple etiologies, such as infection, inflammation, trauma, endocrinopathies, drugs toxicity, and primary metabolic disorders. Metabolic myopathies can be due to inherited disorders of glycogen metabolism or to defects of fatty acid oxidation. Primary respiratory chain dysfunction is a rare cause of myoglobinuria, but it has been described in sporadic cases with mutations in genes encoding cytochrome b or cytochrome c oxidase (COX) subunits and in four cases with tRNA mutations. We describe a 39-year-old woman with myalgia and exercise-related recurrent myoglobinuria, who harbored a novel mitochondrial DNA mutation at nucleotide 4281 (m.4281A>G) in the tRNA-isoleucine gene. Her muscle biopsy revealed ragged-red and COX-deficient fibers. No deletions or duplication were detected by Southern blot analysis. The m.4281A>G mutation was present in the patients muscle with a mutation load of 46% and was detected in trace amounts in urine and cheek mucosa. Single-fiber analysis revealed significantly higher levels of the mutation in COX-deficient (65%) than in normal fibers (45%). This novel mutation has to be added to the molecular causes of recurrent myoglobinuria.

Exertional dyspnea in mitochondrial myopathy: clinical features and physiological mechanisms

Exertional dyspnea limits exercise in some mitochondrial myopathy (MM) patients, but the clinical features of this syndrome are poorly defined, and its underlying mechanism is unknown. We evaluated ventilation and arterial blood gases during cycle exercise and recovery in five MM patients with exertional dyspnea and genetically defined mitochondrial defects, and in four control subjects (C). Patient ventilation was normal at rest. During exercise, MM patients had low Vo(2peak) (28 ± 9% of predicted) and exaggerated systemic O(2) delivery relative to O(2) utilization (i.e., a hyperkinetic circulation). High perceived breathing effort in patients was associated with exaggerated ventilation relative to metabolic rate with high VE/VO(2peak), (MM = 104 ± 18; C = 42 ± 8, P ≤ 0.001), and Ve/VCO(2peak)(,) (MM = 54 ± 9; C = 34 ± 7, P ≤ 0.01); a steeper slope of increase in ΔVE/ΔVCO(2) (MM = 50.0 ± 6.9; C = 32.2 ± 6.6, P ≤ 0.01); and elevated peak respiratory exchange ratio (RER), (MM = 1.95 ± 0.31, C = 1.25 ± 0.03, P ≤ 0.01). Arterial lactate was higher in MM patients, and evidence for ventilatory compensation to metabolic acidosis included lower Pa(CO(2)) and standard bicarbonate. However, during 5 min of recovery, despite a further fall in arterial pH and lactate elevation, ventilation in MM rapidly normalized. These data indicate that exertional dyspnea in MM is attributable to mitochondrial defects that severely impair muscle oxidative phosphorylation and result in a hyperkinetic circulation in exercise. Exaggerated exercise ventilation is indicated by markedly elevated VE/VO(2), VE/VCO(2), and RER. While lactic acidosis likely contributes to exercise hyperventilation, the fact that ventilation normalizes during recovery from exercise despite increasing metabolic acidosis strongly indicates that additional, exercise-specific mechanisms are responsible for this distinctive pattern of exercise ventilation.

Reproducibility and absolute quantification of muscle glycogen in patients with glycogen storage disease by 13C NMR spectroscopy at 7 Tesla.

Carbon-13 magnetic resonance spectroscopy (13C MRS) offers a noninvasive method to assess glycogen levels in skeletal muscle and to identify excess glycogen accumulation in patients with glycogen storage disease (GSD). Despite the clinical potential of the method, it is currently not widely used for diagnosis or for follow-up of treatment. While it is possible to perform acceptable 13C MRS at lower fields, the low natural abundance of 13C and the inherently low signal-to-noise ratio of 13C MRS makes it desirable to utilize the advantage of increased signal strength offered by ultra-high fields for more accurate measurements. Concomitant with this advantage, however, ultra-high fields present unique technical challenges that need to be addressed when studying glycogen. In particular, the question of measurement reproducibility needs to be answered so as to give investigators insight into meaningful inter-subject glycogen differences. We measured muscle glycogen levels in vivo in the calf muscle in three patients with McArdle disease (MD), one patient with phosphofructokinase deficiency (PFKD) and four healthy controls by performing 13C MRS at 7T. Absolute quantification of the MRS signal was achieved by using a reference phantom with known concentration of metabolites. Muscle glycogen concentration was increased in GSD patients (31.5±2.9 g/kg w. w.) compared with controls (12.4±2.2 g/kg w. w.). In three GSD patients glycogen was also determined biochemically in muscle homogenates from needle biopsies and showed a similar 2.5-fold increase in muscle glycogen concentration in GSD patients compared with controls. Repeated inter-subject glycogen measurements yield a coefficient of variability of 5.18%, while repeated phantom measurements yield a lower 3.2% system variability. We conclude that noninvasive ultra-high field 13C MRS provides a valuable, highly reproducible tool for quantitative assessment of glycogen levels in health and disease.

Oxygen-induced regulation of the intracellular glutathione levels in erythrocytes

We are replying to Dr. Böning’s comments to our recently published paper on oxidative stress parameters at moderate altitude (Heinicke et al. 2009). Half cell redox potential for the couple reduced (GSH)-oxidized (GSSG) glutathione is widely accepted as a reliable measure of the intracellular redox state (Schafer and Buettner 2001). Alterations in the intracellular GSH levels may reXect the change in its de novo production, reduction of GSSG or total oxidized glutathione (R-SSG) to GSH as well as the shifts in its oxidation rate. Several groups, including the co-authors, have previously shown that hypoxia (in vivo and ex vivo) results in an increase in GSH content in rodent and human erythrocytes (Bogdanova et al. 2003, 2009). The question raised by Dr. Böning is on the origin of a transient increase in the GSH levels in human erythrocytes after accent to the altitude of 2,800 m. He suggests that at least some of the hypoxia-induced increase in GSH may be explained by its liberation from deoxygenated hemoglobin (Hb). We disagree that the GSH data measured in our study are partially “artifacts”, as we were using a reliable method of assessment of GSH, and the data could be reproduced a suYcient number of times, thereby allowing reliable statistical analysis. This observation is in line with the experimental data of ours and those of Dr. Böning’s group (Hütler et al. 2000). The only technical diVerence in sample preparation between our group and that of Hütler et al. is that they used perchloric acid for Hb precipitation. Whereas, we have chosen a precipitation solution especially developed for erythrocytes that preserved GSH from oxidation in Hb-rich environment (Beutler 1971). Based on this technical diVerence, we suggest that an increase in GSH in erythrocytes subjected to deoxygenation indeed takes place. However, we are still convinced that deoxygenation decreases free radical production in erythrocytes, although it is not the case in some other tissues. Our recent data indicate that endothelial NO synthase (RBC-eNOS) is one of the major contributors to the *O2-generation, and regulation of the GSH levels in erythrocytes (Mihov et al. 2009). Its activity is O2-dependent, and deoxygenation will lead to its deactivation. In line with this observation, met-Hb levels were lower in erythrocytes equilibrated with gas mixtures containing 5–0.5% O2 compared to that in equilibrium with 21% O2 in L-arginine/nitrite-free solutions (Bogdanova et al. 2009). Therefore, it is rational to assume that deoxygenation leads to a decrease in oxidative load in erythrocytes. Would GSSG assessment be a reliable marker of oxidative stress in our human in vivo experimental settings? Not necessarily. GSH levels in blood were assessed in human blood over the days during the assent. It is known that acute oxidative stress indeed causes increased GSSG levels before depletion of the GSH pool is observed. However, when accumulating in the cytosol GSSG above the threshold Communicated by Susan Ward.